1. Field of the Invention
The present invention relates to a method of liquefying natural gas.
2. Description of the Background
The liquefaction of natural gas has been carried out for many years for the purpose of storing the same for later use and for reducing the volume thereof so that it can be economically transported.
Various refrigeration cycles have been used to provide the refrigeration required to liquefy natural gas. One typical refrigeration cycle used is a cascade refrigeration cycle employing three individual refrigerants in series, each of which is circulated in closed cycle in heat exchange relationship with the feed stream and with each other. This type of cycle is relatively efficient but has a high capital cost due to the fact that numerous heat exchangers, compressors and interconnecting pipelines are required.
Another refrigeration cycle used for liquefaction of natural gas employs a multicomponent refrigerant fluid which is first cooled by heat exchange with cooling water or ambient air and is then totally condensed and subcooled by heat exchange with the same multicomponent refrigeration stream after it has been expanded to low pressure. At any given temperature of the low pressure multicomponent refrigeration stream excess refrigeration is produced which is used to liquefy the natural gas. Many variations of this type of refrigeration cycle have been used such as using one or more partial condensations, separating the liquid from the gas after each partial condensation and remixing the condensed fractions at low pressure to reconstitute the original stream. In another variation, the multicomponent refrigerant stream is first cooled and partially condensed by a single component refrigerant stream circulating in a closed cycle.
Some of the variations of refrigeration cycles which have been used may be found, for example, in U.S. Pat. Nos. 3,020,723, 3,645,106, 3,763,658, and 4,065,278.
U.S. Pat. No. 3,020,723 describes a liquefaction method and apparatus wherein the refrigeration cycle is partitioned into separate stages whereby use is made of separate refrigerants in areas where the refrigerants are most effective as a heat exchange medium.
U.S Pat. No. 3,645,106 discloses a closed cycle refrigerant, wherein the multicomponent refrigerant is compressed and then successively fractionated by partial condensation in a plurality of steps to provide condensates at progressively decreasing temperature levels. The condensates are separated and introduced under reduced pressure into a common zone in heat exchange with the natural gas and vaporization of the condensates. The multicomponent refrigerant is withdrawn from the zone for recycle.
U.S. Pat. No. 3,763,658 pertains to a refrigeration system wherein a feed stream is first subjected to heat exchange with a single component refrigerant in a closed, cascade cycle. Then, the feed stream is subjected to heat exchange with a multicomponent refrigerant in a multiple zone heat exchange forming a portion of a second, closed refrigerant cycle.
Finally, U.S. Pat. No. 4,065,278 describes a liquefaction process in which feedstock is isentropically expanded and distilled at a pressure lower than the critical pressure to form an overhead rich in methane and a bottom fraction. In this method, the methane rich overhead is compressed utilizing the energy obtained from the expansion and then the compressed overhead is liquefied in a refrigeration cycle.
The advantage of the multicomponent refrigeration cycles is a low capital cost due to the few pieces of equipment that are required. On the other hand, the power required is higher than for a pure component cascade cycle.
Natural gas is predominately methane but also contains many other components such as ethane, propane and other hydrocarbon gases and water vapor, carbon dioxide and nitrogen. The quantity of nitrogen in the natural gas can vary widely. Typical natural gases may contain anywhere from nearly zero percent up to 10 percent or more. It is desirable to remove the nitrogen from the natural gas during the liquefaction thereof to reduce the concentration of nitrogen in the liquid collected in the storage tank. Nitrogen in the liquid natural gas takes up volume and reduces the amount of methane and other combustible gases that can be stored. Moreover, nitrogen in the liquid natural gas reduces its temperature and increases the refrigeration required for liquefaction. Further, even small concentrations of nitrogen of on the order of 1% in the liquid natural gas can induce stratification of the liquid natural gas in a storage tank into distinct layers. The lower layer can store heat for a period of time and then quickly mix with the upper layer, releasing the heat by suddenly vaporizing a large quantity of natural gas. Thus, it is desirable to remove the nitrogen from the natural gas before it is stored rather than design the system to safely handle the periodic release of a large quantity of vaporized natural gas from the storage tank.
A natural gas liquefaction plant using a multicomponent refrigeration cycle typically has at least two compressors. The multicomponent refrigeration cycle requires a compressor to circulate the multicomponent cycle gas from low pressure to high pressure. A second compressor is required to compress boil off gas generated in the liquid natural gas storage tank due to heat leak into the tank. The compressor also compresses any flash gas generated when the liquid natural gas enters the tank. A compressor for the feed gas may also be used but frequently the natural gas feed is available from a pipeline at sufficient pressure to be liquefied.
Conventionally, small quantities of nitrogen have been removed from liquefied natural gas by allowing sufficient vaporization to occur as the nitrogen enters the storage tank so that most of the nitrogen leaves with the flash gas and the remaining liquid quantity contains only a small quantity of nitrogen. In order to remove larger quantities of nitrogen, however, the same has been removed by distillation during the liquefaction process.
A need continues to exist, therefore, for a method by which nitrogen can be removed from liquefied natural gas in an efficient manner requiring the use of low power.